Did an experiment in a school's heat system. Turned off the unit ventilator valves all at once. The pipe pressure raised significantly causing the circulation pumps to back off. Some notes, this is a reverse return loop, hot water flowing (not steam), unit ventilators are 2-pipe. Question: why did pressure increase when uni-vents were throttled? I thought Bernoulli said that narrowing pipes would cause velocity to increase and pressure to decrease...shutting off all branches should narrow all flow to the supply line only, right?